The accurate positioning of patients and tissue localization are crucial steps before performing many types of medical treatments. One category of medical treatments in which the proper placement and verification of the position of patient or tissue is of particular importance is in the field of radiation therapy.
Radiation therapy involves medical procedures that selectively expose certain areas of a human body, such as cancerous tumors, to high doses of radiation. The intent of the radiation therapy is to irradiate the targeted biological tissue such that the harmful tissue is destroyed. To minimize damage to surrounding body tissue, many conventional treatment methods utilize “dose fractionating” technique. According to “dose fractionating” technique, radiation dosage is delivered in a planned series of treatment sessions with each delivers only a portion of the total planned dosage. Healthy body tissue typically has greater capacity to recover from the damage, thus reducing the amount of permanent damage to healthy tissue while maintaining enough radiation exposure to destroy tumoral tissue.
The efficacy of “dose fractionating” procedure depends in large part upon the ability to irradiate the exact same position on a body over multiple radiation sessions. The goal is to place the patient or body parts of the patient in a way that the target area is in the same position relative to the radiation source in each and every treatment session. Inaccuracies in positioning could result in errors in radiation dosage and treatment locations, leading to unpredictable disease relapse or damage to healthy tissues. However, problem arises when doctors attempting to recreate the same target area of a patient at every radiation session.
The traditional approach to control patient positioning is to place marks or tattoos at specific locations on the patient's skin. Several laser or light sources from predetermined locations project beams of light at the patient's body. To control the patient positioning, a therapist moves the patient until the marks are aligned with the lines from the lasers or light sources. A significant drawback to this approach is that the accuracy and consistency of the patient positioning is heavily dependent upon the skill level of the therapist in manually positioning the patient. In addition, with heavier patients, it is possible that only the skin of the patient is moved into the proper position without moving the body part to be irradiated into the appropriate position. Moreover, this approach does not provide an efficient way to record and reflect the positioning quality in the patient's records. Shortcoming of this approach may be fully illustrated by problems faced in external-beam radiotherapy (EBRT). EBRT is a commonly used curative treatment for a variety of cancers including prostate cancer. The efficacy and tolerance of EBRT depends on accurate localization and the maximal exclusion of critical normal structures, such as organs at risk, with conformal field arrangements. However, the position of the target is typically inferred only from skin markings, which serves as external references points, which are used to align the patients on the treatment machine. The accuracy of this alignment is monitored with periodic portal imaging of the skeletal structures of the lower pelvis. Although external reference points should be reproducibly related to skeletal structures, portal imaging often reveals displacement from the intended position, and this is often referred to as setup variation. In prostate cancer EBRT, investigators demonstrated that the error in field alignment might exceed 10 mm along each of the mutually perpendicular axes of the coordinate system. Vigneault E, Pouliot J, Laverdiere J, et al. Electronic portal imaging device detection of radio-opaque markers for the evaluation of prostate position during megavoltage irradiation. Int J Radiat Oncol Biol Phys 1997; 37:205-212. Tinger A, Michalski J M, Cheng A, et al. A critical evaluation of the planning target volume for 3-D conformal radiotherapy of prostate cancer. Int J Radiat Oncol Biol Phys 1998; 42:213-221. Stryker J A, Shafer J, Beatty R E. Assessment of accuracy of daily set-ups in prostate radiotherapy using electronic imaging. Br J Riol 1999; 72:579-583. Zelefsky M J, Crean D, Mageras G S, et al. Quantification and predictors of prostate position variability in 50 patients evaluated with multiple CT scans during conformal radiotherapy. Radiother Oncol 1999; 50:225-234. The setup variation only partially accounts for uncertainties in the position of the target relative to the treatment beam. The position of the prostate is not fixed relative to the skin marks or the skeletal anatomy of the pelvis. The state of bladder and rectal filling may result in displacement of the prostate. Roeske J C, Forman J D, Mesina C F, et al. Evaluation of changes in the size and location of the prostate, seminal vesicles, bladder, and rectum during a course of external beam radiation therapy. Int J Radiat Oncol Biol Phys 1995; 37:205-212. Beard C J, Kijewshi P, Bussiere M, et al. Analysis of prostate and seminal vesicle motion: Implications for treatment planning. Int J Radiat Oncol Biol Phys 1996; 34:451-458. This organ motion adds an additional component of uncertainty, which may also exceed 10 mm, in localizing the target for treatment delivery.
Another approach to control patient positioning is to utilize an immobilization device to maneuver the patient into a particular position. An immobilization device physically attaches to the human body to keep the patient from moving once proper positioning is achieved. A drawback of using an immobilization device is that such devices do not exist for all body parts. Immobilization devices are generally effective only for positioning the head and neck of a patient. Moreover, in many known immobilization devices, a patient can still move to a significant degree within the confines of the immobilization device. In addition, these devices can be extremely uncomfortable for the patient. Furthermore, the immobilization has little or no control over aforementioned organ movement.
To solve these problems, researchers have developed a real-time electronic portal imaging device procedure to identify gold markers located within the patient's body and correct daily variations in target position during external beam radiotherapy for prostate cancer. Gold intraprostatic markers are implanted around the target of interest in the patients through CT-guided transgluteal approach or a transscrectal ultrasound-guided perineal approach and are used to localize internal reference points. The researchers found that a direct match to the markers provides an easy, time-efficient, and more reliable method than using portal skeletal images and obviates the need to calculate the centroid. M. G. Herman, T. M. Pisansky, J. J. Kruse, et al., Technical aspects of daily online positioning of the prostate for three-dimensional conformal radiotherapy using an electronic portal-imaging device, Int. J. of Radiation Oncology Biol. Phys., 57:4, pp 1131-1140 (2003). J. M. Schallenkamp, M. G. Herman, J. Kruse, T. M. Pisansky, Prostate position relative to pelvic bony anatomy based on intraprostatic gold markers and electronic portal images, Int. J. of Radiation Oncology Biol. Phys., accepted for publication Feb. 17, 2005. However, because the markers used in these experiments are designed for other purposes and are mostly cylindrical in shape with smooth surfaces, the same study also revealed that the fiduciary markers might change positions over time.
Thus, there is a need for better designed markers and method that improve attachment markers to the target sites within a patient and thus minimizes marker migration after implant.